This invention relates to a method of separating inorganic sulfides from carbonaceous material such as coal, coke, oil shale, and other carbonaceous products. Inorganic sulfides include pyritic sulfur, that is sulfur in the form of pyrite or marcasite. For the most part, FeS.sub.2 makes up about 40-80% of the total sulfur in coal with the remaining sulfur combined with organic compounds.
The inorganic sulfur is present in macroscopic and microscopic forms. Consequently, physical separation of pyritic sulfur from coal has required crushing to a very fine size in order to liberate the microscopic pyrite which may be contained in domains as small as one or two microns in diameter. Ordinary, specific gravity separations have been effective only to remove the coarser pyritic sulfur from coal and other carbonaceous materials.
One approach in overcoming these problems is described in U.S. Pat. No. 3,807,557 to Miller, one of the present co-inventors. Finely divided coal is formed into an aqueous pulp and subjected to froth flotation to float and remove most of the carbonaceous material from the coarse pyritic material in the underflow. The underflow also contains clay and mineral shale. The carbonaceous material in the froth is repulped, conditioned with a coal flotation depressant and a pyrite flotation collector to float the fine-size pyrite while removing the coal product as underflow in the second flotation stage.
Coal flotation depressants have been selected from organic colloids, for instance a carbohydrate such as dextrin or modified carbohydrates, i.e., modified corn or potato starch. Other colloid depressants include proteinaceous material, such as glue, gelatin, albumin, casein or whey. In addition, a complex polyhydroxycarboxylic acid or a gluccide of high molecular weight such as quebracho extract, tannin, or saponin have been suggested. A convenient source of coal depressant is Aero Depressant 633, a modified soluble carbohydrate available from the American Cynamide Company.
The high costs of these depressants and agents used in the earlier processes have made the two stage flotation process for removing sulfur from coal uncompetitive with the current practices involving use of low sulfur coal, petroleum fuel and other low-sulfur fuels.
Therefore, in view of these considerations, it is an object of the present invention to provide an improved process for removing pyritic sulfur from coal.
It is a further object to provide a method for depressing carbonaceous material during pyrite flotation through the use of a coal derived depressant.
It is also an object to provide a coal depressant that is derived from coal or other carbonaceous materials.